


RADical Birthday

by ShadowedAphelion



Category: Undertale (Video Game)
Genre: HAPPY FUCKING BIRTHDAY YOU NEON BASTARD!, Knotting, Mating Cycles/In Heat, Other, Oviposition, Parasitism, Vaginal Penetration, Vaginal Sex, eggs baybeeeee!, slight body horror, you know
Language: English
Status: Completed
Published: 2020-04-21
Updated: 2020-04-21
Packaged: 2021-03-02 00:28:53
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,021
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23766169
Author URL: https://archiveofourown.org/users/ShadowedAphelion/pseuds/ShadowedAphelion
Summary: Just a little something I whipped up from a twitter poll for Fresh!Sans' birthday. Totally uneditied. Maybe rushed. TECHNICALLY, IT's STILL 4/20!!!
Relationships: Sans/Sans (Undertale)
Comments: 7
Kudos: 177





	RADical Birthday

**Author's Note:**

> Enjoy, you dirty, dirty, dirty skelesinners.

**RADical Birthday**

**Dom!Fresh/Sub!Error**

* * *

  
Note: Fresh is They/Them because you have the parasite, and the skeleton host.

Contains: Mentions of parasitism, slight body horror. Mating cycles/In heat, Knotting, Vaginal penetration, Oviposition.

* * *

“Heyyyy, Error, broski! How's it hangin’ my radtastic world wrecker?” Error jolted from his perturbed observations of some boring replicated timeline, expression going from bored to mildly wary. Turning on his heel, the glitched skeleton affixed the 90’s looking nightmare with a patented “I hate everything” expression.

Fresh was well known to be rather unflappable, even in the face of certain death. Sure, Error was fully capable of breaking an entire universe over his knee in a fit of impotent rage. Or how Nightmare could easily impale several monsters and scatter their dust like confetti in a manner of seconds. But Fresh- Fresh was another type of weird when it came to physical attacks.

“What do you want?” Error snapped, scowling heavily when the taller skeleton rolled over on his heelys, despite the fact that he carried a neon cyan skateboard in his arms. He leaned away, almost subconsciously when the 90s monstrosity casually slung an arm over his shoulder. 

“So angry already, brah?” Fresh asked, their voice filled with a strange, disconnected sort of amusement. There was a dark plum tint to their cheekbones, and even through his overcoat, Error could still feel the immense heat practically radiating out of them. “You’s always buggin’ out, can’t a guy just visit an old pal?”

“You’re no pal of mine, you’re an annoyance and a fu **[n]** king cockroach.” Error scoffs, wriggling a bit to try and get out of Fresh’s hold. “And stop censoring my swear words, you and I both know da **[ng]** well that you’re no goody two shoes, you belligerent, neon-wearing **[broski]**.”

Fresh’s ever present smile seemed to falter at the edges, before their expression changed to something that Error could describe as menacing. The neon blue and yellow “YOLO” on their glasses abruptly changed to a darker, more purple colored “SHUT”. 

Error was too proud to admit it, but the sight of the other’s expression changing so fast caused fear to jolt up his spine. “Now, Error. Try to keep it calm with all dat UNRAD lingo, ya got me, buddy?”

“...Right” Error agreed after a moment, only relaxing when that toxic purple reverted back to their typical bright colors. And then after another moment of silence. “Sorry.”

“Dun worry, it’s cashed!” And just like that, Fresh was back to smiles again as if it never happened. Error twitched a little at that, frowning to himself. “Now, I know that ‘cha hate to hear it, but I need your help with somethin~”

“And what would that be?” Error asked, immediately suspicious. “You know that I can just warp away whenever I want right?”

“Dat’s true, but remember Error-” Fresh laughed, their smile growing a tad too wide, and once again their expression started to slip. Error muttered a very soft expletive when he was suddenly manhandled until they were practically eye to eye. 

Fresh’s glasses were slipping down their face, and Error swallowed nervously when he realized that there was no soul in the other’s eye-socket. It simply didn’t make sense, the parasite was still there. _(It even hissed at him, the dark purple of it’s mantle flushed an odd pink)_. It was almost as if-

-one of Fresh’s tentacles gently caressed his face. Error shuddered at the sensation of slime cooling against his cheekbone, unable to look away as Fresh’s smile became way too smug for the glitches taste. When had Fresh gotten so… emotional? “Ya owe me, yanno, for that one thing. Ya wouldn’t be lyin’ ta me, cause that’s pretty **_fucking_ ** rude of ya.”

Ah, well. How can Error say no to something that fucking terrifying?

* * *

“You want to do _what_ with me?!” Error all but shouted in disbelief, before he found himself practically squeaking as Fresh all but drop-kicked the shorter glitch into some abandoned timeline. 

They both landed in a clumsy pile of limbs and tangled up in each other’s clothes, bouncing briefly on something soft. The most infuriating part, Error realized, was that Fresh’s glasses managed to stay on through the whole thing as they gracelessly untangled themselves from each other. The same thing couldn’t be said for that stupid fucking backwards snapback the bastard wore. 

Fresh’s head was cocked to the side as they gazed down at the glitch whom they had pinned beneath their lanky body. Eye-lights strangely bright in the darkness of wherever they ended up in. Error swallowed nervously, especially when he realized that he could clearly see the other blink behind the usually pitch dark lenses. Error could even see the true form staring down at him, and for a moment, he could’ve sworn that he saw some sort of ravenous hunger in their gaze.

Creepy, but it brought a strange flush to Error’s cheekbones. Now that he was so close to the taller, he once again noticed that blazing warmth that radiated off of the other’s body. In fact, he could pick up the sounds of Fresh’s breathing having a sort of panting quality to it. 

“I said that I wanna _hit_ ya. Y’see, This body o’ mine’s _blazin’_ with heat. Can'tcha feel it?” Fresh panted a little, grinning wider as they settled themself down upon the other, caging them with lanky limbs while Error squawked in protest.

“What do you mean _hit?_ Speak english for the stars’ sake!” Error spat out, feeling his face somehow get even warmer now that he was being pinned down by the other’s weight.

“I wanna fuck ya, broski.” Fresh said with a snicker, the sound turning to an almost cackle at Error’s deer-in-the-headlights expression when the parasite once again cursed. “Do the horizontal tango, knock hips, take the one-eyed snake to the optometrist, put the hawt dawg into the bun-” 

“Please stop talking.” Error gasped, wriggling a little before ultimately giving up. “Why me? Couldn’t you just ask Ink-”

“We ain’t letting that soulless rainbow bastard touch us.” All mirth left Fresh’s voice, and Error faltered at the suddenly blank expression of the other. “Inky’s no friend of ours- not after what he did ta-”

Fresh went silent for a moment, briefly sitting up to mess with their glasses before smiling down at Error once more. “-Eh, whatever, it don’t matter right now.”

As much as Error wanted to ask, he refrained from it. Not when Fresh was obviously so unstable. They waited in awkward silence before Error eventually sighed. 

“If- If you fuck me,” Error could feel his blush growing brighter after the words escaped his teeth. “Then that means that the debt is- repaid?”

“Yuppers,” Fresh replied with a surprisingly wry grin. “At least until the next time I save that sweet butt of yours.”

“Shut up, that was a fluke and you know it.” Error bit back, but most of the fight was clearly gone from his voice. His body also felt warm, especially with the close proximity. Now that he could focus on it, Error swallowed nervously when he realized that he could feel so much of the other. “Fine.”

Fresh made the strangest purring trill noise, expression brightening visibly behind dark lenses. The look that crossed the symbionte’s face was almost cute in a dorky sort of way. Error stilled for a moment as the other briefly got off. Fresh’s facial maxilla and zygotic bones were glowing a bright, vivid lilac in the almost black of the room. Fresh seemed to be trembling a little, eagerness evident throughout their whole form. It was strange, Error noted distantly, to see how emotive the monster had seemed to have gotten since Error's last seen the terror.

_(Hadn’t their clothes changed too? Though, the neon crocs and socks combo was somehow even worse than the last set of shoes they wore.)_

With little fanfare, Fresh unzipped their windbreaker and tossed it somewhere that Error couldn’t see, and then went to work on kicking off their shoes and pants. With each loss of cloth, the room gradually brightened as purple magic was revealed.

No wonder why Fresh seemed so heavy, the other had his ecto summoned the entire time. The other’s heat must’ve made the other’s magic significantly more dense. Error sighed a little shakily, before awkwardly fumbling with his own clothes, pulling off his shorts and dropping them off to the side. He did the same with the overcoat, then went to remove his sweater, only pausing when Fresh suddenly spoke up. “Ey, Glitches, leave the top on, kay?”

“Why?” Error asked, and faltered when Fresh smiled and revealed that their distal phalanges had shifted to claws. Error felt his mouth grow dry, and he croaked out a simple ‘oh’.

“So I dun scratch the shiznits outta ya, duh.” Fresh said with a hum, voice almost sassy. “Unless ya like that sorta thing~”

As appealing as the thought was, Error really didn’t want any permanent marks on his form. The thought of _his_ scarf getting torn up wasn’t on Error’s to-do list, even though taking it off left him feeling naked and vulnerable.

_(Somehow even more so, even though Error was now only in the dark blue woolen sweater)_

Fresh made another one of those trilling purrs, and Error felt the smallest thrill run down his spine as the other settled themselves down between Error’s legs. Fresh wiggled a little in eagerness, the words on his glasses read “HAWT” as they moved Error’s body until the shorter was pressed flush against the taller. 

Fresh's hands were surprisingly gentle, the purr singing through the taller chest. Error inhales shakily as Fresh runs their fingers across his sensitive bones, and a soft moan escapes his teeth as the other coaxes the midnight blue skeleton's magic to manifest. 

“Yer sooo bodacious, Glitchy~” Fresh chirps, and Error makes a scoffing sound, face flushing bright blue when the other goes to nuzzle their nasal ridge against Errors cheekbones.

“I can’t even feel flattered.” Error groans, though he _hates_ how he can hear the slightest note of fondness in his own, staticky tone. “You and your stupid words…”   
  
A soft chittering laugh escapes Fresh, Error pauses when he feels the warm bone of Fresh’s hand tilting up his skull so he can look socket-to-socket and is pulled into a kiss. Error’s hands are shaking a little as he reaches up to grasp at the firm magic of Fresh’s forearms, almost clinging to the other. Their teeth click together, and Error can only moan softly as the taste of what could only be passionfruit of all things, overtakes his senses. Their tongues tangle, Fresh’s purring ramping up into a rolling rumble as Error’s sockets close, leaning in. 

Fresh’s hands continue to wander, slipping under the blue of Error’s sweater, mindful of their sharp claws as they knead at the supple magic of Error’s ecto. Error breaks the kiss, a mix of violet purple spilling down his chin as he gasps for breath. A high-pitched keen escapes the glitch when Fresh finds the other’s breasts, one claw circling the nipple, while thumbing at the soft areola with a thumb. 

Error shifts a little, rocking his body a little into Fresh’s, a soft little noise of surprise when he feels something hot and wet press itself against the cooler flesh of his summoned cunt. He grips the taller tightly, whining when it cards itself between the plump blue magic, slicking up Error’s already dripping pussy. One multi-colored hand trailing down Fresh’s bare chest, and down to feel at the tentacle-like summoned between the two of them. Fresh shudders against him as Error thumbs at the pre-soaked slit of Fresh’s dick, a soft groan escaping the taller. 

“Please-” Error gasps, sounding needy as he continues to rock and grind himself up into Fresh’s warmth. “Please fuck me.”

“Mm~ Soon.” Fresh moans in agreement, panting heavily into Error’s neck, vibrant purple tongue lapping at the sensitive bone of Error’s cervical vertebrae.

Error makes a despairing sound when Fresh removes themselves from his sweater, only to squeak when Fresh grips his thighs. The fatty magic of his ecto bulging between the gaps of the others fingers as they all but drag Error into their lap. Error finally catches sight of the thing that had been teasing his entrance, swallowing nervously at the thick, purple colored tentacle tinted violet with Errors own fluids. Fresh makes a soft, barely there humming sound, and Error frowns in confusion when the other doesn’t make a move to- well, _move._

Error glances down at himself, then back up to the parasite with a perturbed expression. “Well? Wh-what are you waiting for, **[Broski]**?”

Fresh smiled a little, expression oddly shy before positioning Error above their dick, visibly shuddering in excitement as the tip gently prodded at the glitched skeleton’s soaked entrance. Aaaand they let go of Error’s hips.

“H-holy-” Fresh gasped as Error’s warm and pliant body sank down surprisingly easily onto their cock, the shorter skeleton whimpering loudly. Fresh’s phalanges gripped at the fabric of Error’s sweater in a tight knuckled grip with a low, rough noise of satisfaction. “-Schnikes, Error, you’re so _tight_.”

Error merely gasped wordlessly below, multicolored bones fisting at the sheets with trembling hands. A soft whine escaped the glitched skeleton, soft walls fluttering around the thick tentacle of Fresh’s cock as it bottomed out deep inside of his soaked cunt. They sat there for a brief moment of time, holding onto each other tightly as they both allowed themselves to adjust. 

“Sooo tight.” Fresh’s voice was breathless, the letters on their shades flickering between a fast assortment of nonsensical letter combos as purple-tinted drool dripped down their chin. Their body trembled a little as they gained their bearings, before slowly rolling their hips up into Error. “We can- _hhn_ -feel your pretty pussy squirmin’ all around us, brah.”

“Shut up!” Error finally managed to find the voice to speak, voice slightly shaky and unstable, with little crackles of static. The midnight blue skeleton’s facial maxilla and zygotic bones flushed a bright blue interspersed with flecks of brilliant gold. “D-don’t say it- like that you fu **[n]** king **[rad dude]**!”

Fresh merely laughed in a manner that made Error blush even brighter, and the glitch could only whine and bury his face into Fresh’s chest with a frustrated noise of embarrassment. Error could feel Fresh’s hands palming at the squishy flesh of his outer thighs, and he made a muffled noise of surprise when the neon-wearing monstrosity began to start a slow, if not clumsy rhythm. 

Only, that clumsiness and awkward pace didn’t last long, gradually smoothing out into an easy sort of temps as the taller found their rhythm. It took an even shorter time for Error to start moaning and gasping in pleasure as Fresh starts to alternate their pace into a baffling mix of shallow and then deep thrusts. 

Error could only groan, whining into Fresh’s shoulder as the taller assisted him in bouncing on their cock. Fresh had utterly filled him, thick, heavy, and hot, cock slick with a mix of Error’s own slick and the parasite’s pre-come. Error mewled sweetly, gasping for breath, blue drool spilling down his jaw at the sensation of the tentacle twisting and rubbing against his innermost walls. It felt- so unbelievably good. 

Error’s sockets were burning at the corners, tears threatening to spill down his cheeks as he felt the steadily tightening coil deep in his gut tensing with every wet thrust and smack of the taller’s hips against his soaked thighs. Fresh was muttering odd noises into his neck, their fangs scraping at his vertebra, breath hot and heavy. 

“A-Ah-” Error cried out, hands scrabbling for purchase on Fresh’s sides, eye-lights almost rolling up into his skull as he came. Fresh growled something absolutely filthy, but Error could barely understand it over the buzz that overcame his skull. 

* * *

Fresh snarled deep and low inside their chest as Error came undone in their arms. Error looked so pretty like this, face flushed a vivid blue, those pretty star-like freckles of magic across the bridge of his nasal aperture and cheekbones stark. How they could see the faint purple of their magic beneath Error’s rich blue ecto. How his entire body tensed up. 

Fresh hugged Error tightly to themselves, growling and purring roughly as they continued to thrust up and deeper still into Error’s ever tightening and constricting heat. 

Fresh hissed lowly, letters on their lenses briefly read “MINE” as they felt Error’s cunt twitch and spasm around their dick, pulling them deeper into the shorter’s wet heat. Their fangs sharpened, but Fresh refrained the feral urge to sink their teeth into the shorter’s cervical vertebra and claim the shorter right then and there. They were only here to take an edge off their heat, not to force Error into an unwanted bond. So they settled for nipping at the woolen fabric of Error’s sweater instead, drooling hot purple. 

With a breathless sounding gasp, Error’s mismatched eye-lights rolled to meet their hidden ones. Expression briefly blitzed out and hazy as he weakly pawed at their upper arms, he mewled softly, and Fresh couldn’t help themselves, leaning into to capture the other into another kiss.

Error wasn’t as responsive this time, but Fresh didn’t mind as they continued to thrust their hips, even as Errors cunt fluttered around them. Still, Error eventually came back to his senses, lazily deepening the kiss with his multiple tongues. 

Fresh was careful to maneuver their position until Error was mostly lying back against the mattress. Error groaned a little, the multicolored skeleton instinctively wrapping his legs around their middle, plush thighs soft against their hips. Error stared up at them, expression placid yet pleasured, and Fresh couldn’t help themselves, the sight sending a bolt of heat straight to their very core. 

They all but picked up their pace, Error whining beneath them and Fresh reached out to tangle Error’s fingers up in their own. Surprisingly enough, Error didn’t seem to mind, linking their digits together, moaning softly and sweetly. Fresh panted loudly, drool spilling from the corner of their mouth as the quiet air was filled with the sounds of Error’s moans, Fresh’s own purring, and the wet noises of their joining.

Error was still so wonderfully tight, even after cumming heavily around their cock. He just felt, so nice and sweet, pleasantly cool against the offending heat that battered Fresh’s senses. Like a moment of brief clarity. Or maybe even a bottle of ice water after a day at the skatepark. They almost wished that they could do this sort of thing forever, but it wouldn't be fair to either of them. 

The pleasure building in their body was getting stronger and stronger. And they could feel Error starting to reach his peak again.All Fresh could do was continue to watch Error’s expression and sounds. Taking every little noise he made, how his tongues flopped out of his mouth. How his bones were slick with their perspirants, faintly visible in the ecto-light of their bodies. 

“Ahnn- shiiiiiit-” Fresh groaned out, the little curse filling their insides with giddiness. They continued to fuck Error quickly and without misery, and finally, they could feel themselves pressing against something, burying themselves deep into Error as the other came with a scream. 

They followed with their own scant seconds afterwards, The symbiont let out a hissed groan as they felt their pace start to fall apart, but Error didn’t seem to care. Pussy quivering around them, squeezing and milking them for all they were worth. Fresh’s fangs glittered in the low light, a feral snarl escaping them as they bit into the collar of Error’s sweater. 

Their cock twitched deep in the recesses of Error’s body, writhing against velvety walls as he knotted the other with a lewd sounding pop. Fresh made a strange, clicked keening noise, the letters on their glasses briefly flickering out as they flooded Error with come. The shorter one made a gasped noise, staring at the ceiling. Error hand clumsily reached over to wrap around Fresh’s neck when the other shuddered as they tried to get a look at his face.

Their teeth clashed together, even as Fresh’s hips continued to jitter, knot swelling further as it plugged Error’s cunt full. Error tasted like blue raspberry, but more sweet than tangy. Fresh twitched, an off sensation making their insides twist, forcing them to break the kiss.

  
Luckily, Error didn’t seem to mind, too fucked to care and content.

Fresh hiccuped a little, grimacing as they felt something shift, and they pressed a shaking hand against their middle. Magic was gathering in their body, until it spread down to their cock. Fresh trembled for a moment, the heat receding yet sinking lower. Then, they felt something move deep within them, drawing a low punched out sound.

Something hot settled into their gut, and Fresh whimpered, hips subconsciously rocking even though they had nowhere else to go. They fisted the sheets, shutting their eyes tightly, gradually the warmth seemed to start traveling, even as it drew a soft whine from their teeth. 

They let out an undignified yelp, and Fresh could only stare uncomprehendingly for a moment, eye-lights briefly widening behind their glasses at the sight of Error’s ecto magic unconsciously shifting to accommodate whatever was happening. It looked like a strange chamber, and whatever it was, it triggered something instinctive in Fresh’s body. Fresh gasped a little, palming at their own stomach as pain briefly overtook the pleasure in their body. They cringed, panting heavily even as Error made a hazy noise of confusion. Fresh shuddered a little at the sensation of the strange internal movements. Until something eventually started to slide their way down the bulk of their cock, how they pressed against something deep inside Error, and eased their way in with little fanfare.

“O-Oh.” Fresh said after a moment, breathless and horrifically embarrassed at the sight of _eggs_ slowly filling up the shorter’s ecto. “Ooooh.”

“Oops.”

* * *

“P-please t-tell me that you didn’t just-” Error gasped out from where he lay curled against Fresh’s side. His head was hazy from pleasure, but he could still feel the round weights in his ectoic stomach shifting whenever he breathed or moved. Fresh made a soft, embarrassed sound, nuzzling into his neck. 

“We’re sorry.” Fresh squeaked out, even as the purr continued in their chest as they gently rubbed at the taut magic of Error’s belly. The eggs were clearly visible beneath the magic, colored a soft purple from a mix of come and their own color, the colors swirling beneath deep indigo. “They’re just duds, so ya don’ gotta worry. They’ll sorta, yanno, dissolve into your stores inna few days-”

“You egged me.” Error said blankly after a moment, sluggishly turning his head to the side to half-heartedly glare at the other. “I can’t believe you. I cannot fu **[n]** king believe that you just egged me-”

“We- I” Fresh stuttered, but Error beat them to this embarrassment salad, tossing an arm over his arm to hide his burning face.

_“And I liked it.”_ Error hissed, slapping at Fresh with their freehand. He affixed Fresh with his patented ‘I hate you so much right now, you belligerent, neon-looking head ass looking fucker’ expression as he gave them a hearty smack against their sternum. “This is your fault, you always drag me into weird shit, I hate you.”

“Bruh, we said that we’re sorry-” Fresh protested with a wheeze, both from the hit and the embarrassment. “-do you want us to like, skedaddle?” 

“I swear to every single deity that maybe out there, if you even think of leaving me like this, I’ll **[freaking]** kill you!” Error groaned, scowling at the word censoring. “I wasn’t finished, you **[Rad Dude]** . I-I I don’t mind doing this again, just- warn me next time if there’s weird biology **[schnikes]**.”

  
“...Oh.” Fresh said after a moment, flushing a dark purple and meekly purring. 


End file.
